The present invention relates to a heat-sealable, ovenable container and to a method of manufacturing the container. More particularly, the present invention is directed to a heat-sealable container which is provided with a self-venting, easy-tear, sanitary seal which prevents discoloration during heating and decreases the bacteria, mold, and fungi levels in food sealed within such containers.
The most common containers for convenience foods which are to be heated within the containers are formed of thin sheet aluminum or layers which include aluminum foil. Because of the relatively high cost of such containers and because they generally cannot be used in microwave oven cooking, substantial efforts have been made to provide plastic coated paperboard containers which can withstand oven heating.
Presently, thermoplastic polyethylene terephthalate coated paperboard trays are being used in both microwave and conventional ovens. These trays are equipped with mechanically attached polyethylene terephthalate coated paperboard lids as well as uncoated paperboard lids or alternatively, the trays are sealed with transparent or translucent films. The unsealed mechanically attached lids vent well during rethermalizing, that is, heating food stored within the container, and readily accept detailed art work. However, since the lid and tray are not a unitized sealed container, package stability and hygiene problems are encountered when trays containing food are stored and/or stacked. Thus, color changes frequently occur in food which is rethermalized in unsealed containers.
On the other hand, although translucent film lids also provide effective self-venting during rethermalization, containers or cartons containing film-type closures or lids are generally fragile and thus break easily and accordingly are not stackable. Also, such containers perform badly with respect to their acceptance of detailed art work and ingredient copy. Furthermore, since films are transparent or at best translucent, any color change or formation of condensation which may have occurred during freezing, thawing, or cooking is readily noticeable through the film. Since frozen foods in the frozen state are not very attractive until reconstituted to an edible condition, it is desirable not to package foods so as to be visible in the retail package.
The present state of the art for forming food containers involves sealing thermoplastic polyethylene terephthalate coated paperboard lids to thermoplastic polyethylene terephthalate coated paperboard trays with heat before cooking. However, because the polyethylene terephthalate, which acts as a hot melt adhesive, is coated on both the lid and the tray, the seal produced therebetween is too strong and thus the container cannot be readily opened after rethermalization without damaging the container or its contents. Furthermore, although the seal is strong, it is not water-tight which disadvantageous from a hygienic point of view. Additionally, present sealing methods cannot seal thermoplastic polyethylene terephthalate lids to trays in which the polyethylene terephthalate has already been crystallized by heat. This crystallization is a common phenomenon since the food is cooked in the tray at elevated temperatures up to about 425.degree. F. which is just about at the crystallization range of polyethylene terephthalate.
Accordingly, an object of the present invention is to provide a heat-sealable container which protects food contained therein by decreasing the bacteria, mold and fungi levels within the container.
Another object of the present invention is to provide a heat-sealable, ovenable container which is provided with a self-venting, easy-tear seal between the lid and the tray of the container.
A further object of the present invention is to provide a heat-sealable, ovenable paperboard container which readily accepts detailed art work and accordingly can be printed by gravure, lithography, or flexography.
Still another object of the present invention is to provide an improved method of achieving a seal between the lid and the tray of an ovenable container which is effective in achieving all of the aforementioned objects.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the present invention will become apparent to those skilled in the art from this detailed description.
Pursuant to the present invention, the above disadvantages may be eliminated and the objects of the present invention can be achieved by providing one portion of the container, for example, the lid with a thermoplastic polyester coating and the other portion of the container, that is, the tray with a cross-link thermosetting polyester containing coating. On the other hand, if the lid is provided with the cross-linked, thermosetting polyester-containing coating, then the tray must be provided with a thermoplastic polyester coating. It is the unique combination of a thermoplastic polyester coating and a cross-linked, thermosetting polyester coating on respective portions of a container which is effective in achieving a seal which provides a sanitary closure during handling and shipment, prevents discoloration of the food during heating, decreases bacteria, mold, and fungi levels in the food disposed within the container and is self-venting upon the substantial completion of the cooking or reheating cycles. Thus, when the sealed container is heated, an increase in internal vapor pressure acts as a barrier in preventing dehydration and caramelization of the food contained therein. When the release of the seal takes place, that is, just about the time the cooking cycle or reheating cycle is completed, the product is ready to be served. Since the vapor barrier is present in the carton or container up to this point in time, none of the aforementioned problems are produced. Thus, the use of the seal container as defined by the present invention retains heat since no steam is released during the heating process. The retention of the heat and the moisture drastically reduces the amount of bacteria, mold, and fungi levels of the prior art containers.